I have also tried deleting mrmc data, uninstalling, reinstalling etc etc...nothing has made any difference. The only way of getting movies to play is to either just use the Plex server as a UPnP Source, or create a separate library in MrMC referencing back to the files on the Nas. Obviously this isn't ideal as I am then maintaining 2 libraries!

Thanks for the article link, I will have to read through and check my router settings.

I am not sure why it works on plex app and not on MrMC.... try manually logging in using user/pass in Settings->Services -> Plex and see if that helps.

you sure that transcoding is not enabled in Settings->Services -> Plex?

Is the DNS rebinding issue not more applicable to the IP address of the device rather than the apps that device is running?

Just as an experiment, I tried a similar exercise on my Android phone connected over wifi on my home network. ie ran a stream from the Android Plex App, which ran fine, and showed as a local stream. Then installed MrMc on the same device, signed into the plex client and tried to run a movie through this. SD movies played fine, but they showed as a remote stream on the Plex Server dashboard. Higher bitrate 1080 and 4k movies failed to play.

So this is the exact same behaviour as on the Nvidia Shield TV.

Any further ideas what else may be causing mrmc to be seen as a remote device?